Packaged device and method of forming same

ABSTRACT

A method of packaging an integrated circuit die ( 12 ) includes the steps of loading an array of soft conductive balls into recesses formed in a platen and locating the platen in a first part of a mold cavity. A second part of the mold is pressed against the balls to flatten a surface of the balls. A first mold compound then is injected into the mold cavity such that the mold compound surrounds exposed portions of the balls. The balls are removed from the platen and a first side of an integrated circuit die is attached to the balls such that the die is surrounded by the balls. Die bonding pads on a second side of the die are electrically connected to respective ones of the balls surrounding the die, and then the die, the electrical connections, and a top portion of the conductive balls is encapsulated with a second mold compound. The result is an encapsulated IC having a bottom side with exposed balls.

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuit packaging, and moreparticularly to an improved process for fabricating a Ball Grid Array(BGA) packaged device.

An integrated circuit (IC) die is a small device formed on asemiconductor wafer, such as a silicon wafer. A leadframe is a metalframe that usually includes a paddle that supports the IC die that hasbeen cut from the wafer. The leadframe also has lead fingers thatprovide external electrical connections. That is, the die is attached tothe die paddle and then bonding pads of the die are connected to thelead fingers via wire bonding or flip chip bumping to provide theexternal electrical connections. Encapsulating the die and wire bonds orflip chip bumps with a protective material forms a package. Depending onthe package type, the external electrical connections may be used as-is,such as in a Thin Small Outline Package (TSOP), or further processed,such as by attaching spherical solder balls for a BGA. These terminalpoints allow the die to be electrically connected with other circuits,such as on a printed circuit board. However, it can be expensive andtime-consuming to form a leadframe and package a device if steps likechemical etching and etch back are required.

Thus, it would be desirable to eliminate such etching steps, andfurther, to eliminate the need for the leadframe. It is desirable tohave a method of decreasing the size of packaged ICs too.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of preferred embodiments of theinvention will be better understood when read in conjunction with theappended drawings. The present invention is illustrated by way ofexample and not limited by the accompanying figures, in which likereferences indicate similar elements.

FIG. 1 is an enlarged cross-sectional view of a packaged integratedcircuit in accordance with a first embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view of a packaged integratedcircuit in accordance with a second embodiment of the present invention;

FIG. 3 is an enlarged cross-sectional view of a packaged integratedcircuit in accordance with a third embodiment of the present invention;

FIGS. 4A-4H are perspective views illustrating the steps for fabricatingpackaged semiconductor devices in accordance with the present invention;

FIGS. 5A and 5B are cross-sectional views illustrating the formation ofa plurality of packaged semiconductor devices in accordance with thepresent invention;

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of the presently preferredembodiments of the invention, and is not intended to represent the onlyform in which the present invention may be practiced. It is to beunderstood that the same or equivalent functions may be accomplished bydifferent embodiments that are intended to be encompassed within thespirit and scope of the invention. Further, it will be understood bythose of skill in the art that the drawings have been simplified and arenot drawn to scale.

The present invention provides a method of packaging an integratedcircuit die, including the steps of:

loading a plurality of conductive balls into recesses formed in aplaten;

transferring the platen to a cavity of a first part of a mold;

pressing a second part of the mold against the conductive balls, therebyat least partially flattening a top surface of the conductive balls;

injecting a first mold compound into the mold, wherein the first moldcompound surrounds exposed portions of the conductive balls;

removing the conductive balls from the platen, wherein the conductiveballs are held together by the first mold compound;

attaching a first side of an integrated circuit die to a plurality ofthe conductive balls, wherein the die is surrounded by others of theconductive balls;

electrically connecting die bonding pads on a second side of the die torespective ones of the conductive balls surrounding the die; and

encapsulating at least the second side of the die, the electricalconnections, and at least part of the conductive balls with a secondmold compound.

In another embodiment, a heat sink may be located coplanar with theconductive balls and the die attached to a surface of the heat sink.

The present invention further provides a packaged integrated circuit diehaving a plurality of conductive balls having a top, flattened surfaceand a first mold compound that holds the plurality of conductive ballstogether. The top, flattened surfaces of the balls are generallycoplanar with a top surface of the first mold compound, and bottom,opposing surfaces of the balls are exposed. An integrated circuit diehaving a first side is attached to a plurality of the conductive balls.The die is surrounded by others of the conductive balls. The die iselectrically coupled to the others of the conductive balls. A secondmold compound encapsulates at least a second side of the die opposingthe die first side, and the top surfaces of the conductive balls.

Referring now to FIG. 1, an enlarged cross-sectional view of a packagedsemiconductor device 10 formed in accordance with an embodiment of thepresent invention is shown. The packaged device 10 includes a pluralityof conductive balls 12 having a top, flattened surface 14 and bottom,opposing surface 16. A first mold compound 18 holds the plurality ofconductive balls 12 together. The top, flattened surfaces 14 of theballs 12 are generally coplanar with a top surface of the first moldcompound 18, and the bottom, opposing surfaces 16 of the balls 12 areexposed. An integrated circuit die 20 has a first side thereof 22attached to a plurality of the conductive balls 12. The die 20 issurrounded by others of the conductive balls 12 and the die 20 iselectrically coupled to the others of the conductive balls 12. A secondmold compound 24 encapsulates at least a second side 26 of the die 20opposing the die first side 22, and the top surfaces 14 of theconductive balls 12. The second mold compound 24 is of a type known tothose of skill in the art that is commonly used for packaging integratedcircuits.

The conductive balls 12 preferably are C5 (Controlled Collapse ChipCarrier Connection) solder balls. The balls 12 are made with anelectrically conductive material so that electrical signals can bepassed therethrough. The conductive material may be a metal, such ascopper or gold, or an alloy thereof, or a soft solder, such as 63/37material (63% tin, 37% lead) or 90/10 (90% lead, 10% tin).Alternatively, the soft balls 14 may be formed of a polymer basematerial with metallic surface finish. A lead free package can beachieved by using lead free metallic balls.

The bottom surfaces 16 of the balls 12 are exposed and allow theintegrated circuit die 12 to be connected to a printed circuit board.The balls 12 are used to pass data, power and ground signals to/from thedie 20 and a substrate or printed circuit board (not shown), as is knownby those of skill in the art, while others of the balls 12 to which thedie 20 is attached can conduct heat from the die 20 (i.e., thermalmanagement) and enhance board level solder joint strength. That is, theballs 12 under the die 20 provide a solderable surface that enhancesboard level solder strength. It is noted that balls at the corners andeven along the package edges also can provide connections that enhancethe mechanical strength of the package.

The top surfaces 16 of the balls 12 are flattened and coplanar. The topsurfaces 16 may be flattened by pressing, as discussed in more detailbelow. The balls 12 are held together with the first mold compound 18.The first mold compound 18 is of a type known to those of skill in theart that is commonly used for packaging integrated circuits.

The integrated circuit die 20 may be any type of known die, such as onecut from a silicon wafer, and may be any type of circuit, such as adigital signal processor, a special purpose circuit, etc. Such circuitsare well known to those of skill in the art. The bottom surface 22 ofthe die 20 is attached to the top flattened surfaces 14 of one or moreof the conductive balls 12 with a die attach material (not shown). Thedie attach material may be an adhesive of a type well known in thesemiconductor art for attaching dies to substrates, such as an epoxy.

The top surface 26 of the integrated circuit die 20 includes a pluralityof die bonding pads 28. The die bonding pads 28 are electricallyconnected to ones of the conductive balls 12 that surround the die 20.In one embodiment, the die bonding pads 28 are connected to theconductive balls 12 with wires 30 via wirebonding. Known wirebondingprocesses, such as thermosonic wire bonding may be used to connect thewires 30 to the die bonding pads 28 and the top surface 14 of the balls12.

As is known by those of skill in the art, various size wires areavailable for connecting die to substrates, with the wire size beingselected based on, among other things, pad pitch. The bond wire 30 has adiameter of between about 15 μm to about 55 μm, although other diameterbond wires may be used and the invention should not be limited to aparticular bond wire diameter. For example, 63 μm pitch applications use25 μm diameter wire, while 52 μm and 44 μm pitch applications use 20.3μm diameter wire. Development is being done for a 37 μm pitchapplication using 17 μm diameter wire. Thus, as is known in the art,fine pitch and ultra-fine pitch wire bonding processes may be performeddepending on the pad pitch. In one embodiment of the invention,insulated bond wire is used. Insulated bond wire comprises a conductivecore coated with an electrically insulating material and is suitable forfine pitch and ultra-fine pitch wirebonding. The insulating materialprevents the wire from shorting to other wires or other conductivestructures. In general, gold and aluminum are the most commonly usedelements to make the conductive core of the bonding wire. Both gold andaluminum are strong and ductile and have similar resistance in mostenvironments. Gold wire is sometimes doped with a dopant, such asberyllium, calcium in order to stabilize it. Small-diameter aluminumwire is often doped with silicon or sometimes magnesium to improve itsbreaking load and elongation parameters. In addition to gold andaluminum, copper, palladium-alloy, platinum and silver bonding wire arealso known. In an embodiment of the invention, insulated wires having adiameter of less than about 25 μm are used. The insulative coatingpreferably is an organic insulative coating having a thickness of about0.5 μm to about 2.0 μm that can be thermally decomposed during free airball formation. Further, the wires 30 preferably have a meltingtemperature (Tg) of about 180° to 350° C.

The balls 12 are made of a soft material so that the wires 30 can bereadily wirebonded thereto. Preferably, the wires 30 are made of astiffer or more rigid material, such as copper or a hard gold alloy sothat during wirebonding, the wires 30 penetrate into and are embeddedwithin the balls 12. Using a harder material for the wires 30 allows thewires 30 to penetrate into the balls 12, hence forming a connection byembedding the hard wires 30 in the softer balls 12.

Referring now to FIG. 2, another embodiment of a packaged device 32 isshown. In this embodiment, instead of attaching the die 20 to the balls12, the die 20 is attached to a heat sink 34. The heat sink 34preferably is coplanar with and surrounded by a plurality of the balls12. More particularly, a first surface of the heat sink 32 to which thedie is attached is coplanar with the top surface of the balls 12 and asecond, opposing surface of the heat sink 34 is coplanar with the bottomsurface 16 of the balls 12. Thus, the second side of the heat sink 34 isexposed. The heat sink 34 preferably has a thickness that is equal to adiameter of the balls 12 as measured from the top, flat surface 14 tothe bottom surface 16. The die 20 may be attached to the heat sink 34with a die attach adhesive or solder (not shown). In order to providegood thermal properties, the heat sink 34 preferably is formed of asolid metal plate, such as copper slug. The heat sink 34 furtherimproves the thermal performance and increases the solderable area ofthe package 32.

Referring now to FIG. 3, yet another alternative of a packaged device 36is shown. The packaged device 36 is similar to the packaged device 32(FIG. 2) except that a second mold compound 38 has been formed over thedie 20, wires 30 and top surfaces 14 of the balls 12 as a glob top. Inorder to perform the glob top encapsulation process, dams or flanges 40are formed at the sides of the package to retain the second moldcompound 38. The dams 40 may be formed of the first mold compound 18. Itwill be appreciated that the glob top encapsulation process may beperformed on a package with or without the heat sink 34. That is,although the packaged device 36 includes the heat sink 34, the heat sink34 is not required.

Referring now to FIGS. 4A-4H, a method for packaging an IC die inaccordance with an embodiment of the present invention will bedescribed. FIGS. 4A and 4B are enlarged cross-sectional viewsillustrating a platen or fixture 50 into which a plurality of conductiveballs 12 is loaded. The platen 50 is a block of metal having a largeflat surface with a plurality of cavities or recesses 52 in which theballs 12 are loaded. The cavities 52 are laid out in a grid or array onthe surface of the platen 50. FIG. 4A shows the balls 12 being loadedinto the platen 50 and FIG. 4B shows the balls 12 disposed within thecavities 52. The cavities 52 are generally circular, as shown. However,the cavities 52 may have a flat or partially flattened bottom surface sothat the bottoms of the balls 12 can be flattened or partiallyflattened, such as by pressing. The balls 12 may be loaded into therecesses 52 using known technology, such as a shaker. The recesses 52are spaced at a pitch to match the required ball pitch for a packageddevice, such as 1.27 mm apart. Once the balls are loaded in the recesses52, the entire platen 50 is transferred into a mold cavity. Analternative to using the platen 50 is to load the balls 12 into an arrayof holes located in the bottom of a mold cavity. The balls 12 could beplaced in such holes in a number of ways, such as with a mask. FIG. 4Cshows the balls 12 being placed in a platen 54 that has recesses 52 anda space 56 for a heat sink 34. In this embodiment, the heat sink 34 isplaced in the space 56 prior to loading the balls 12 into the recesses52.

As previously discussed, the balls 14 are formed of a soft, conductivematerial that can be deformed, such as a soft metal. Example metals aresolder or gold. As is known by those of skill in the art, most BGA ballsare made out of 63/37 material (63% tin, 37% lead), which is very soft.Even when 90/10 (90% lead, 10% tin) material is used, the ball shape candeform. Referring now to FIGS. 4D and 4E, after the balls 12 are locatedin the platen 50, the top surfaces 14 of the balls 12 are flattenedeither with a first top 58 of the mold, or by a press, via a mechanicalcoining process. That is, the mold top 58 is pressed against the topsurfaces 14 of the balls 12 within the cavities 52 of the platen 50,which flattens the top surfaces 14 of the balls 12. If the cavities 52also have a flat or partially flat bottom surface, then the bottomsurfaces 16 of the balls 12 are flattened in the same way as the topsurfaces 14. Thus, either one or both of the top and bottom surfaces 14and 16 of the balls 12 are flattened or partially flattened.

Referring now to FIG. 4F, after the mechanical coining process, a firstinjection molding process is performed such that the first mold compound18 surround the balls 12. That is, the first mold compound 18 isinjected into the mold and the first mold compound 18 surrounds theexposed portions of the balls 12. Injection molding is well known in theart and further description thereof is not required for a completeunderstanding of the invention.

Referring to FIG. 4G, after the first mold compound 18 solidifies, thefirst top 58 of the mold is removed. At this point, the balls 12 may beremoved from the platen 50 or the balls 12 may remain in the platen 50for further processing, as described below. For illustrative purposes,in FIG. 4G the balls 12 are shown detached or separated from the platen50. However, in the presently preferred embodiment, the balls 12 are notyet removed from the platen 50.

Referring to FIG. 4H, the integrated circuit die 20 is attached to aplurality of the balls 12 at a predetermined location. The die 20 has abottom surface 22, and a top surface 26 that includes a plurality ofwire bonding pads 28. In the presently preferred embodiment, the bottomsurface 22 of the die 20 is attached directly to the balls 12 with a dieattach adhesive (not shown). The die 20 is attached at locations suchthat the die 20 is surrounded by a plurality of the balls 12. The numberof I/O's is used to determine the number of balls 12 that surround thedie 20. Attaching the die 20 on the top surface 14 of a plurality of theballs 12 allows a platen into which the balls 12 are loaded to beuniform in size and have a single ball matrix pattern. A single ballmatrix pattern allows different size die and different package sizes tobe formed without having to change the size of the platen. Further, asdiscussed above, the balls 12 upon which the die 20 is attached serve asa thermal path for the die 20 and provide a solderable surface thatenhances board level solder joint strength.

As previously discussed, other options for die attach exist. Forexample, instead of a die attach adhesive, the die could be soldered tothe balls 12. Further, as shown in FIGS. 2 and 3, the die 20 can beattached to a heat sink 34 (e.g., a solid metal plate) with an epoxy orsolder. The heat sink 34 can further improve the thermal performance ofthe package and increase the solderable area of the package.

Referring again to FIG. 4H, after the die 20 has been attached to theballs 12, as described above, the wire bonding pads 28 on the die 20 areelectrically connected to respective ones of the balls 12 that surroundthe die 20. More particularly, bonding wires 30 are connected from thedie wire bonding pads 28 to respective ones of the balls 12. One end ofthe wires 30 is connected to the die bonding pads 28 and another end isconnected to the balls 12 using a wire bonding process as is known inthe art. However, as the balls 12 are held together with the first moldcompound 18, in order that the structure of the first mold compound 18is not compromised, an alternative to the ultrasonic force used in thetraditional thermosonic wire bonding process may be used. For example,instead of applying the traditional thermosonic bonding, hard wire madeof copper or a hard gold alloy is used to penetrate into the balls 12,which form connections by embedding the wires 30 into the balls 12. Asshown in FIG. 4H, tips of the wires 30 are embedded within the balls 12.The tips are generally circular in shape and are formed by electricflame off (EFO) of the wires 30 by a wire bonding machine.

Referring back to FIG. 1, after the wirebonding process is performed,the die 20, the electrical connections, the wires 30 and a portion ofthe balls 12 are encapsulated with a second mold compound 24. Again, aknown injection molding process is used. The second mold compound ispreferably a plastic material, as is known in the art for packagingintegrated circuits.

After the encapsulation process, the now packaged die 20 is removed fromthe mold and the platen 50, thereby uncovering the bottom surfaces ofthe array of balls 12, as shown in FIG. 1. To ensure reliableinterconnection performance, the exposed bottom surfaces 16 of the balls12 may be coated or plated with a noble surface finish (e.g., NI/Au)that is both resistant to corrosion and provides low contact resistance.The exposed portions of the balls 12 may be coated via selectivedeposition of electrolytic Ni/Au platings.

Referring now to FIG. 5A, rather than forming a single package, multiplepackages are formed at the same time. FIG. 5A shows three such packages60. The packages 60 are separated by cutting along saw streets 62,preferably using a well-known saw singulation process, to form separatepackaged devices 10, as shown in FIG. 5B. The saw streets 62 are flangesthat are formed of the first mold compound 18 during the first moldinjection process. The saw streets (flanges) 62 protect the balls 12embedded in the first mold compound 18 during die attach, wire bondingand the second mold injection processes.

Prior to the saw singulation process, an optional electrical functionaltest may be performed. Since all of the I/O terminals of theencapsulated devices are separated throughout the packaging process, anelectrical test in strip format is possible, hence improving testerutilization and making parallel testing possible without additionalprocess or cost.

The packaged device has improved high-frequency electrical performancebecause the signal path from the die 20 to the board is shortened.Further, system reliability is improved by increasing solder jointresistance to stress failures caused by deflections of the system board.Improved RF performance and resistance to mechanical stress failures areimportant issues to cellular telephone handset makers.

The present invention provides an easy and inexpensive method ofpackaging an integrated circuit. Two layers of mold compound are used,with the C5 balls being embedded into the first layer of mold compound.The top surfaces of the C5 balls act as bond fingers and the bottomsurfaces provide stand-off on a PCB. A packaged device can include aheat slug for better thermal performance. Further, the device may beformed using a glob top encapsulation. The device cost is low becauseneither a substrate nor terminations (external pins) are required. Sincethere is no metal leadframe, the saw blade used in the singulation stepdoes not have to cut through metal, so the saw blade will have a longerlife. Also, since no leadframe is required, there is no need to performsubstrate trace routing. The packaging process does not require anychemical etch-back, which can be expensive. The packaging process may beperformed using currently available equipment. The package also has avery low profile, down to about 0.4 mm. Although a LGA package isdescribed, other package types, such as QFN (Quad Flat No lead) also canbe formed using the aforedescribed method. Stacked die devices can bepackaged with this method too. Land grid arrays (LGA) offer highinterconnection density, e.g., 200+ I/Os is possible.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not limited tothese embodiments only. Numerous modifications, changes, variations,substitutions and equivalents will be apparent to those skilled in theart without departing from the spirit and scope of the invention asdescribed in the claims.

1. A method of packaging an integrated circuit die, comprising the stepsof: loading a plurality of conductive balls into recesses formed in aplaten; transferring the platen to a cavity of a first part of a mold;pressing a second part of the mold against the conductive balls, therebyclosing the mold and at least partially flattening a top surface of theconductive balls; injecting a first mold compound into the mold, whereinthe first mold compound surrounds exposed portions of the conductiveballs and holds the conductive balls together; removing the second partof the mold to expose the top surface of the conductive balls; attachinga first side of an integrated circuit die to a plurality of theconductive balls, wherein the die is surrounded by others of theconductive balls; electrically connecting die bonding pads on a secondside of the die to respective ones of the conductive balls surroundingthe die; and encapsulating at least the second side of the die, theelectrical connections, and at least part of the top surface of theconductive balls with a second mold compound.
 2. The method of packagingan integrated circuit die of claim 1, wherein a mask is used to transferand place the conductive balls into a bottom cavity of the mold.
 3. Themethod of packaging an integrated circuit die of claim 1, wherein theconductive balls comprise solder balls.
 4. The method of packaging anintegrated circuit die of claim 1, wherein the platen has an array ofrecesses for receiving the conductive balls.
 5. The method of packagingan integrated circuit die of claim 4, wherein the recesses are spaced ata predetermined pitch.
 6. The method of packaging an integrated circuitdie of claim 5, wherein the platen includes a space for receiving a heatsink, the method further comprising the step of placing a heat sinkwithin the heat sink receiving area.
 7. The method of packaging anintegrated circuit die of claim 6, wherein the heat sink has a firstside that is generally coplanar with the flattened top surface of theconductive balls and wherein the die attaching step comprises attachingthe first side of the die to the first side of the heat sink.
 8. Themethod of packaging an integrated circuit die of claim 1, wherein in thedie attaching step, the die is attached to the flattened, top surface ofa plurality of the conductive balls.
 9. The method of packaging anintegrated circuit die of claim 1, wherein the electrically connectingstep comprises wirebonding the die bonding pads to the respective onesof the balls with a corresponding plurality of wires.
 10. The method ofpackaging an integrated circuit die of claim 9, wherein in thewirebonding step, the wires penetrate into the balls and are embeddedtherein.
 11. The method of packaging an integrated circuit die of claim10, wherein the wires are insulated wires.
 12. The method of packagingan integrated circuit die of claim 10, wherein the balls are formed of ametal that is softer than the wires so that the wires are embedded inthe balls.
 13. The method of packaging an integrated circuit die ofclaim 9, wherein the wires are attached to the top, flattened surface ofthe balls.
 14. The method of packaging an integrated circuit die ofclaim 1, further comprising the step of saw singulating the encapsulateddie from adjacent encapsulated die.
 15. The method of packaging anintegrated circuit die of claim 1, wherein the conductive balls areembedded into the first mold compound with the flattened surface of theballs being substantially coplanar with a top surface of the first moldcompound.
 16. The method of packaging an integrated circuit die of claim15, wherein a surface of the conductive balls opposing the flattenedsurface of the balls protrudes beyond a bottom surface of the first moldcompound.
 17. The method of packaging an integrated circuit die of claim1, wherein the encapsulating step comprises performing a glob topencapsulation process.
 18. A packaged integrated circuit die assembledin accordance with claim
 1. 19. A method of packaging a plurality ofintegrated circuit dice, comprising the steps of: loading a plurality ofconductive balls into recesses formed in a platen; transferring theplaten to a cavity of a first part of a mold; pressing a second part ofthe mold against the conductive balls, thereby at least partiallyflattening a top surface of the conductive balls; injecting a first moldcompound into the mold, wherein the first mold compound surroundsexposed portions of the conductive balls; attaching first sides of aplurality of integrated circuit dies on the plurality of the conductiveballs at predetermined locations, wherein the dies are surrounded byothers of the conductive balls; electrically connecting die bonding padson second sides of the dies to respective ones of the conductive ballssurrounding the dies; encapsulating at least the second sides of thedies, the electrical connections, and at least part of the conductiveballs with a second mold compound; and singulating adjacent encapsulateddies to form individual packaged devices.
 20. A packaged integratedcircuit die, comprising: a plurality of conductive balls having top,flattened surfaces; a first mold compound that holds the plurality ofconductive balls together, wherein the top, flattened surfaces aregenerally coplanar with a top surface of the first mold compound, andbottom, opposing surfaces of the balls are exposed; an integratedcircuit die having a first side attached to a plurality of theconductive balls, wherein the die is surrounded by others of theconductive balls, and wherein the die is electrically connected to theothers of the conductive balls; and a second mold compound encapsulatingat least a second side of the die opposing the die first side, and thetop surfaces of the conductive balls.